The present invention relates to flow control arrangements and more particularly to such arrangements utilised within turbine engines.
Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
As can be seen there are a number of fixed structures such as pylons and stator vanes utilised in order to control air flow and also to support casing structures, etc. These structural features create flow distortions further downstream and/or upstream, and these distortions can reduce the stability margin of downstream components. Furthermore, it is known that the onset of instability in terms of rotating stall/surge is triggered by such distortions but is not random but always occurs in a particular location relative to the structure induced distortion.
Conventional approaches to addressing instability in the flow relate to so-called casing treatment in terms of creating casing distortions, that is to say bumps and hollows to adjust and stabilise fan exit flow distortion, etc as well as asymmetrical flow path cross-sections. Such approaches can significantly add to engine complexity and more importantly may reduce engine efficiency.
Stationary distortions usually occur in an otherwise axisymmetric designed device due to structural requirements, such as fan exit flow distortion caused by a pylon. The situation is graphically illustrated in FIG. 2 below. The view is that looking down from the fan 101 tips and the air flows into the engine from the left side. The presence of a pylon 100 causes high pressure in front of it (p+) and further away in the two sides the pressure is relatively low (marked as p−). The pressure field of the pylon 100 may also transmit into the core compressor to induce an inlet flow distortion in that core. A fan and compressor subject to such distortions in general show a reduced stability margin which may endanger the engine during operation. Guide vanes 102 may also be provided and these vanes will add to potential complexity.
In addition to use of passive casing treatments it will also be understood that active control techniques with regard to compressor stabilities can be used whereby specific control elements are adjusted to achieve stability during operation. These control elements may include altering through flap movements the available flow cross-section and also injecting additional control air feeds. These techniques as indicated add significantly to complexity and cost.